In recent years, a microprocessor fabrication technique has been remarkably progressing. On highly integrated circuits, transistors of which number exceeds 600 million are formed. Such an explosive advance is achieved by making the minimum line width of electronic circuits finer, so that lithography using an ArF excimer laser (193 nm) is on its way to being introduced seriously. Hitherto ultraviolet rays and laser light (excimer lasers such as argon fluoride, krypton fluoride and the like) have been used therefor, but in the future, studies will be developed toward commercialization of EUV (extreme ultraviolet) with further shorter wavelengths.
In lithography using the ArF excimer laser, processing of design rules of 0.13 μm or less has been adopted; in which novolac resin and polyvinyl phenol, which are hitherto used on a KrF excimer laser light, can not be used as a resin for use in ArF resist because their absorption properties are strongly exhibited at 193 nm. In view of this, aromatic ones are replaced with aliphatic ones in order to improve transparency, and furthermore, acrylic resins and cycloolefin resins into which a cyclic compound is introduced have been developed in order to ensure etching resistance (Patent Publications 1 to 4).
Acrylic resins can be readily produced and monomers therefor that have a variety of chains have been produced, so that acrylic resins are characterized by providing an extremely high degree of flexibility in molecular design. However, acrylic resins mainly contain a carboxyl group as an alkali-soluble group and therefore high in acidity as compared with phenolic hydroxyl group hitherto adopted in conventional KrF resists, so that it has been difficult to fully prevent a pattern disturbance caused by controlling a dissolution rate in an alkali developing solution and by swelling. From such a background, monomers containing fluoroalcohol-based alkali-soluble groups are suggested (Patent Publications 5 to 7). It is reported that resist materials that use macromolecule compounds containing fluoroalcohol-based monomers as a copolymer component are excellent in resolution performance and swelling-preventing effect, and greatly useful in lithography using a short wavelength light source for the purpose of microfabrication.
On the other hand, a stepper (a reduction projection aligner) has been dramatically improved in resolution performance by performance improvements of reduction projector lenses and by refinements of optical designs. The performance of lenses used in the stepper is expressed by NA (numerical aperture), and its value of around 0.9 is defined as a physical limit in air but it has already been attained at the present time. With this, it is now on attempt to raise NA to 1.0 or more by filling a space defined between a lens and a wafer with a medium having a larger refractive index than air, in which an exposure technique adopting an immersion method in particular use of water has received attention.
In lithography using the ArF excimer laser, it has been pointed out that contact between a resist film and water brings about various concerns. Particularly, acid generated in the film by exposure, changes in pattern shape caused when an amine compound added as a quencher dissolves in water, a pattern collapse due to swelling, and the like are concerned. In view of this, there is given a report that a topcoat layer disposed on the resist is effective at separating the resist film by water.
A topcoat plays an important role in improving the throughput (treatment efficiency). More specifically, in order to run an immersion exposure step with a shorter time, it is required not to leave water on the resist and it is required to enhance the water repellency on the surface also in connection with the number of defects. However, in the case of intending to dissolve the topcoat layer together with exposed portions of the resist at the time of development, a highly water-repellent topcoat is not necessarily high in solubility in developing solution. Rather, the water repellency and the solubility in developing solution are conflicting with each other in most cases. At present, how much improvement is made on the water repellency of a topcoat of a developing solution-soluble type is of utmost concern, in which a variety of attempts are being carried out (Patent Publication 8).
Additionally, attempts to form a finer pattern by using ArF immersion lithography have also been conducted eagerly. One of these attempts is a method adopting a double patterning process, some manners of which have been suggested.
For example, Patent Publication 9 can be cited as a known example of a double patterning process that adopts a freezing process in which a first resist pattern is changed in properties and characteristics so as not to dissolve in a second resist solution. In this example, irradiation with vacuum ultraviolet rays is performed after forming the first resist pattern, followed by freezing. In this method, however, it is indicated that a pattern width of the first resist pattern is changed between prior to and subsequent to the vacuum ultraviolet irradiation; as countermeasures against this, a pattern width correction is made on the first resist pattern with consideration of the pattern width changes caused by the vacuum ultraviolet irradiation. However, a variety of widths and shapes occur in actual fabrication of semiconductors, it is not actually possible to so design the corrected mask as to meet them each.
Of fundamental properties required for the freezing process, the important matter is that the first resist pattern does not dissolve in the second resist solution, and the necessary matter is that the pattern width of the first resist pattern does not change. There are disclosed in Patent Publication 10 a surface treating agent for use in a chemical freezing process, and a pattern formation method using it. However, this method requires operations including: impregnation with the treating agent, subsequent to formation of the first resist pattern; then giving rise to a chemical reaction between the grained treating agent and a resin that exists in the resist pattern; removing an excess of the treating agent that remains on a substrate by rinsing; and removing a rinsing liquid grained into the resist pattern. Since the number of steps is so increased as to make the operations exceedingly complicated, this method is not satisfactory from the viewpoint of productivity.